1. Field of the Invention
This technology relates to charge pump systems.
2. Description of Related Art
In high density memory arrays and nanoscale memory cells, a design requirement is to have the word line (WL) and bit line (BL) voltages ready on standby to perform a fast and accurate read operation. However, this design requirement increases standby current consumption which impacts power requirements on mobile devices with limited battery power.
An example standby pump system has a design goal of generating a stable read voltage VRD that is greater than the supply voltage VDD on standby. However, the generated read voltage is unstable, for reasons described below.
The pump system includes a detector and charge pump circuitry. RDPWR is a big node that connects to many devices, and suffers from high current leakage and heavy loading. However, an accurate read operation requires a stable read voltage.
The charge pump circuitry includes a weaker charge pump periodically enabled by signal ENPMP. The weaker charge pump is active when the detected voltage on the RDPWR node is less than the target read voltage VRD. During standby when awaiting a read command, the voltage variation on the RDPWR node is large, due to accuracy of the voltage detector described below, and the current pulse from the charge pump.
The charge pump circuitry also includes a stronger charge pump. Both charge pumps are enabled to support read current from the RDPWR node during a read operation. The signal ENRD enables the charge pumps when the integrated circuit accepts a read command.
A voltage detector detects the voltage on the RDPWR node. When the RDPWR node has a voltage less than the desired read voltage VRD due to charge leakage from the RDPWR node, then the ENPMP signal enables the weaker charge pump to replenish the leaked charge at the RDPWR node.
Node RDPWR is an example of detect-and-refresh. During the refresh phase, the node RDPWR is connected to an unregulated charge pump output, and is continuously ramping up. If voltage of node RDPWR ramps up too fast, or the voltage detector has slow response time, then the ENPMP pulse width might be too long. Then the voltage of node RDPWR seriously overshoots. The waveform would have large sawtooth.
The voltage detector utilizes a Zener diode or a resistive voltage divider to convert the voltage VRDPWR on the RDPWR node into a voltage VDIV that is less than the supply voltage VDD. A comparator in the voltage detector compares the voltages VDIV and VREF to determine whether to enable the weaker charge pump. However, both the Zener diode and the resistive voltage divider drain DC current from the voltage levels generated by the charge pump. This DC current drain seriously impacts standby current of the integrated circuit, since repeatedly generating high voltage with the charge pump is a heavy power drain.
Further disadvantages in the voltage detector are that the Zener diode device increases process cost, and the large resistors of a resistive voltage divider entail large circuit area, large parasitic capacitance, and slow response time.